Illuminants such as scintillators are used for photon detectors or radiation detectors for detecting gamma-rays, X-rays, α-rays, β-rays, neutron rays and the like. These detectors are widely applied to positron emission tomographs (PET), medical imaging apparatuses such as X-ray CT, various radiation counters for high energy physics, and resource exploration devices, for example.
For example, in the case of a positron emission tomograph (PET), a gamma-ray (annihilation gamma ray: 511 eV) with relatively high energy is detected by coincidence counting, so that a scintillation detector having high sensitivity and exerting a quick response has been employed. Detectors are required to have properties including a high count rate and a high time resolution for removal of noise from random coincidence counting.
Moreover, in recent years, PET called Time-of-flight PET (TOF-PET) has emerged, which involves measuring differences in time required for an annihilation gamma ray to reach a radiation detector, so as to improve the position detection accuracy. A radiation detector to be used for TOF-PET is required to have particularly a quick response, and an important feature of a scintillator to be used for a radiation detector is its short fluorescence lifetime.
In general, a scintillator suitable for these radiation detectors is desired to be a crystal having high density and a large atomic number (high photoelectric absorption ratio) in terms of detection efficiency, and a high light yield, a short fluorescence lifetime (fluorescence decay time) and high transparency in terms of the need for quick response and high energy resolution. A recent system requires a large amount of scintillators to be densely aligned into a long slender shape (e.g., about 5×30 mm for PET) for multi-layering and increasing resolution. Hence, easy handling, workability, capability of preparing a large-sized crystal, and the price are also important factors for selection. In addition, it is also important that the emission wavelength of a scintillator is consistent with the high wavelength range of detection sensitivity of a photodetector.
Recently, a preferable scintillator to be applied to various radiation detectors is a scintillator having a garnet structure. For example, a scintillator having a garnet structure in which emission from the 4f5d level of Ce3+ is used; that is, a Ce-doped (Gd, Y, Lu)3(Al, Ga)5O12 crystal, has been reported (for example, see Patent Document 1 or Non-patent Document 1). It has been confirmed for Ce-doped (Gd, Y, Lu)3(Al, Ga)5O12 that the scintillation properties including density, light yield, and fluorescence lifetime are varied depending on the crystal composition. In particular, a Ce-doped Gd3Al2Ga3O12 scintillator has properties including the density of 6.7 g/cm3 and the light yield of 45000 photons/MeV, and has sufficiently low self-radioactivity, and thus is increasingly applied not only to PET, but also to medical imaging apparatuses such as X-ray CT, various radiation counters for high energy physics, and environmental radiation meters. In the meantime, the relevant scintillator is problematic in its long fluorescence lifetime of about 90 ns.
Moreover, a scintillator containing Gd, Al, and Ga wherein the ratio of the number of atoms Ga/(Gd+Ga+Al+Ce) ranges from 0.2 to 0.3 has been reported (for example, see Patent Document 2). However, when the ratio of the number of atoms Ga/(Gd+Ga+Al+Ce) is 0.3 or less, a Ce-doped Gd3(Al, Ga)5O12 scintillator with the highest performance among Ce-activated garnet scintillators is impossible to achieve single crystal growth by melt growth. Therefore, it is difficult to apply the Ce-doped Gd3(Al, Ga)5O12 scintillator to PET for which a highly transparent large-sized crystal is necessary and high energy physics applications (for example, see Non-patent Document 2).
A garnet scintillator is known to have a crystal structure with 3 sites that are 8-coordinated, 6-coordinated, and 4-coordinated sites. For example, a Ce-doped (Gd, Y, Lu)3(Al, Ga)5O12 garnet scintillator is known such that rare-earth elements, Ce, Gd, Y, and Lu, occupy an 8-coordinated site, and Al and Ga occupy 6-coordinated and 4-coordinated sites. However, it is known for Ce-activated garnet scintillators that the anti-site phenomenon takes place involving partial replacement by rare-earth elements in 6-coordinated and 4-coordinated sites, and partial replacement by Al and Ga in the 8-coordinated site, resulting in the generation of an anti-site-derived defect level between band gaps, Ce3+ 4f5d emission inhibited by the defect level, a lowered light yield, and the generation of a long-life emission component (for example, see Non-patent Document 3).